The present invention relates generally to membranes containing molecular sieves, and to new techniques for fabrication thereof.
Molecular sieves are nanoporous solids which possess small (typically  less than 10 xc3x85) pores which are intrinsic to their molecular structure. These pores can be used to selectively control rates of diffusion of various chemicals through a molecular sieve. Selectivity can be induced and controlled through the size and shape of the pore (i.e., simple xe2x80x9ckey-in-lockxe2x80x9d steric hindrance effects), and/or by including moieties which interact with the chemicals involved in manners which add to the selectivity of the pore. Such materials are used routinely in bulk form for separation of different gases, and for dehydration of a working atmosphere. They also have potential large-scale commercial applications in, e.g., high-temperature separation and selection of intermediate reaction products.
A particularly important subclass of molecular sieves are the zeolites. Zeolites are nanoporous inorganic alumino-silicates with pore diameters typically in the 3-7 xc3x85 range.
The crystalline nature of zeolites means that the pores are well defined in size, without variationxe2x80x94an intrinsic property of the crystal structure. That is, if a crystallographically oriented layer of zeolite crystals is grown, the pores thereof will share a common orientation relative to the growth surface. An orientation of particular interest is one in which the pores provide direct (if not exactly normal) paths through such a layer.
Zeolites are archetypal molecular sieves, partially because they are available with a wide range of pore diameters, and partially because they can be found naturally as a component of certain clays. Statements and claims made concerning zeolites generally apply to a broader range of molecular sieves.
However, the use of bulk zeolites is neither efficient nor effective for most chemical separation procedures. As the material is bulk, separation occurs because the difference in diffusion rates through the nanopores of the zeolite causes a large amount of one of the chemicals being separated to become temporarily trapped in the zeolite pores. This restricts one to use of batch or semi-batch separation processes, which are generally not compatible with a properly functioning continuous flow synthesis process, as is usually desirable for very large scale synthesis.
An example of the areas in which such membranes are potentially useful, if the fabrication limitations can be overcome, lies in the area of large-scale organic synthesis. A common situation is where an intermediate step in the synthetic process produces a variety of isomers, only one of which can be used as feedstock for the next step in the synthesis. This situation is often met by taking the typically high-temperature (perhaps 300-500 C.) intermediate product mixture, cooling it to liquid nitrogen temperatures, and then separating the isomers using fractional distillation, a slow and expensive technique. The desired isomer is then heated back to the processing temperature, and the synthetic process proceeds.
The cost of this separation step in energy alone is several tens of millions of dollars per year for a single plant, and the need for the additional apparatus to support the cooling, distillation, and reheating steps can represent a significant portion of the capital cost of the synthesis plant. Clearly there is a need for a substitute process with increased energy efficiency and lower capital cost. Thin film zeolite membranes could fill this role, if suitably defect-free and robust membranes could be fabricated.
One approach toward relieving the processing constraints involved with use of bulk zeolites for separation is to somehow make membranes which comprise zeolites, in which the zeolites can still function effectively as separators (i.e., active zeolites). Although it is in principle possible to create nanoporous materials from source materials which are not naturally porous (e.g., porous silicon, or acid-etched alloy sheets), present fabrication techniques therefor usually do not provide product with adequate selectivity for most practical applications.
Traditional approaches toward synthesis of thin film membranes comprising zeolites generally fall into three main classes. Self-supporting membranes can be made, essentially using sintering techniques. However, such membranes are not durable, and are difficult to adhere to a supportive substrate. Alternately, it is possible to grow single crystal zeolite films on suitable substrates. Unfortunately, such films are highly defective when made in sizes suitable for industrial applications.
The most intensively pursued approach toward fabricating thin film membranes which comprise active zeolites is to grow defective zeolite films (usually polycrystalline in structure), and attempt to fill in the defects using secondary growth of zeolites atop the original layer, or by depositing carbonaceous material in the defects. The result to date has been thick films with blocked flow channels leading to poor penetration flow rates, which have limited thermal stability and problems in adhering to suitable substrates.
Specific attempts to make satisfactory zeolite membranes deserve some description, as such point out the intractable practical problems which have appeared in past work. The publications discussed below are included in the Information Disclosure Statement for the instant application.
The zeolite most studied in the quest for robust selective membranes is ZSM5, which is an orthorhombic crystal type. The chemical formula is Nan[AlnSi96xe2x88x92nO192] with about 16 H2O of hydration, and n less than 27. The pores are aligned along the [010] and [100] crystallographic directions, and have sizes just over 5 xc3x85. The equivalent material with n=0 is sometimes called silicalite.
Chiou et al. (Journal of Materials Science Letters 15, 952-954 (1996)) describe a technique to fabricate continuous zeolite films and membranes. Continuous ZSM-5 films were hydrothermally grown on anodic alumina substrates. The hydrothermal crystal growth process is commonly used in this area of endeavor. The technique used by Chiou et al. is a typical one, although the results of such growth can depend strongly on the growth parameters chosen.
Chiou et al. carry out hydrothermal growth of ZSM-5 by forming a growth solution of tetraethylorthosilicate (TEOS), sodium hydroxide, tetrapropylammonium bromide (TPABr), and de-ionized water so as to obtain the molar ratios 5Na2O :10TPABr:100SiO2:10000H2O. This growth solution and an anodic alumina substrate were sealed in a teflon-coated stainless-steel autoclave. Film growth was then induced by heating the autoclave to 180-200 C. for 1-4 days under autogenous pressure without stirring. The samples were then cooled, washed, and dried.
The result found by Chiou et al. was growth of a thick (about 30 microns) and nominally continuous layer of randomly oriented ZSM-5 crystallites. The lack of crystallite orientation and the extreme thickness makes such a layer inappropriate for use in chemical separation. The arrangement of crystallites makes it likely that large numbers of large pores, cracks, and other interstitial defects exist in these filmsxe2x80x94again destroying any potential for significant chemical selectivity in these materials.
C. Bai et al. (Journal of Materials Science 105, 79-87 (1995)) reported growth of chemically selective silicalite membranes on xcex3-alumina substrates. The layers, with thicknesses of about 5 microns, were grown from a gel on the surface of a tube.
The gel used in the growth process was prepared as follows. 0.95 grams of NaOH, 2.1 grams of TPABr, and 10 grams of finely divided silica were dissolved in 125 grams of distilled water. This solution was then mixed thoroughly and allowed to age for a day before being transferred to and sealed within a xcex3-alumina tube. The tube was placed in an autoclave, and heated to 180 C. under autogenous pressure for 12 hours. Repeated film growth steps were required to produce a nonpermeable membranexe2x80x94that is, one in which permeability appears only because of the pores, and not because of micro and macro defects.
As grown, the membrane pores were blocked by TPABr molecules. To remove these, the membrane was calcined in air. A very slow temperature ramp (about 0.1 C./minute) up to a temperature over 450 C. was required so that the calcining process did not crack the membrane. Significant, although not dramatic, levels of both physical and chemical selectivity were displayed by these membranes. However, the requirement for multiple growth steps, and the observed tendency to crack easily on changes in temperature, suggest that these membranes are unlikely to withstand the rigours of a commercial synthesis plant.
Lovallo and Tsapatsis (American Institute of Chemical Engineering Journal 42, 3020-3028 (1996)) report on their fabrication of a preferentially oriented submicron silicalite membrane. They deposited a precursor layer of zeolite nanocrystals, then used these nanocrystals as growth nuclei in more conventional hydrothermal growth.
A solution of silicalite nanocrystallites (about 1000 xc3x85 in size) were made by aging a standard growth solution as described above. Precursor films are then grown from a mixture of the above solution and a solution of alumina nanocrystallites. The resulting films were dried at 110 C., and calcined at between 550 C. and 750 C. for several hours. The resulting precursor films are polycrystalline and randomly oriented.
Secondary growth of the crystallites in this precursor film in a conventional hydrothermal growth environment results, under the published conditions, in preferential growth of platelike crystallites which grow together to form a continuous and intergrown layer. The crystallites making up this final layer are preferentially oriented. Unfortunately, both types are oriented parallel to the membrane surface, rather than the desired perpendicular orientation.
The above described efforts toward fabricating a zeolite membrane have concentrated on producing a continuous zeolite membrane by multiple zeolite growth cycles. Either hydrothermal growth or a combination of hydrothermal and sol-gel growth processes were used, but the material being grown has always been the desired zeolite.
Y. Yan et al. (Journal of Membrane Science 123, 95-103 (1997)) have investigated the possibility of xe2x80x9ccaulkingxe2x80x9d cracks and other micro defects in a polycrystalline zeolite layer with carbonaceous deposits. They refer to this procedure as xe2x80x9ccokingxe2x80x9d. A ZSM-5 membrane is grown much as described earlier, with an additional step of applying a diffusion barrier to the substrate surface so that the hydrothermal growth solution cannot penetrate into the substrate, thereby preventing formation of a layer which is too thick. Following growth, a large aromatic hydrocarbon is applied to the membrane. This hydrocarbon is too large to enter the ZSM-5 pores, but small enough that all microdefects in the membrane layer should be filled. At this point, the hydrocarbon is carbonized by exposing the membrane to temperatures of about 500 C. It does appear that this procedure is successful at closing many microdefects. However, there is also a large decrease of permeability through the zeolite pores, suggesting that this approach also closes such pores, despite the inability of the hydrocarbon to penetrate said pores. When this difficulty is combined with the probable lack of thermal cycling stability induced by differential thermal expansion between the coke and the zeolite membrane, it appears unlikely that this approach will routinely result in commercially useful membranes.
The present invention enables fabrication of suitable thin-film zeolite membranes by a combination of growth of zeolite crystallizes on a substrate followed by embedding said crystallites in a densified sol-gel product layer. The result is a robust, thermally stable, and highly selective membrane suitable for many chemical synthetic process steps.
The invention is of a new class of composite zeolite membranes and techniques for their fabrication. These membranes have a layer of zeolite crystallites on top of a porous substrate. The cracks and voids between the zeolite crystallites, which essentially eliminate the chemical selectivity which can be exhibited by zeolite nanopores, are filled in by a densified sol-gel product deposited by a sol-gel technique. The result is a rugged, highly selective membrane with good throughput.